The selective action of certain mutagens and carcinogens in reverting frameshift mutations bearing specific sequences is investigated by physical chemical experiments. The interaction of mutagens with self-complementary oligonucleotides of defined sequence (corresponding to known frameshift sites) in solution is monitored by optical and magnetic resonance spectroscopy in order 1) to establish whether the in vivo selectivity corresponds to the binding affinity and intercalative capacity in solution, and 2) to attempt to define the mechanism of this specificity in terms of the equilibrium and structural features of the intercalation complexes in cases found to be active or inactive.